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4.7. Default Result

If none of the mechanisms match and there is no "redirect" modifier,
then the check_host() returns a result of "Neutral", just as if
"?all" were specified as the last directive. If there is a
"redirect" modifier, check_host() proceeds as defined in Section 6.1.

Note that records SHOULD always use either a "redirect" modifier or
an "all" mechanism to explicitly terminate processing.

レコードは明示的に処理を終了するために必ず“redirect”変更子か“all”機構を用いるべきである（SHOULD）。

For example:
v=spf1 +mx -all
or
v=spf1 +mx redirect=_spf.example.com

たとえば：

v=spf1 +mx -all

または

v=spf1 +mx redirect=_spf.example.com

4.8. Domain Specification

Several of these mechanisms and modifiers have a <domain-spec>
section. The <domain-spec> string is macro expanded (see Section 8).
The resulting string is the common presentation form of a fully-
qualified DNS name: a series of labels separated by periods. This
domain is called the <target-name> in the rest of this document.

Note: The result of the macro expansion is not subject to any further
escaping. Hence, this facility cannot produce all characters that
are legal in a DNS label (e.g., the control characters). However,
this facility is powerful enough to express legal host names and
common utility labels (such as "_spf") that are used in DNS.